Many micro-electromechanical systems (MEMS) require an encapsulation under vacuum or under a controlled atmosphere and pressure in order to ensure either a good performance or an acceptable lifetime of operation. The encapsulation has to be performed without the deposition of sealing material on the MEMS device, which can cause damage to the device.
The most popular approach is based on wafer bonding. Here, the sealing is performed by connecting two wafers (device wafer and capping wafer) together by means of a reflowable material.
Alternatively, encapsulation can be done by the fabrication and sealing of surface micro-machined membranes. The use of conformal LPCVD (low-pressure chemical vapor deposition) films is a known method for encapsulation at low pressure. The sealing of the cavity comprising the MEMS devices is done while depositing the conformal film. Hence, the atmosphere and pressure of the sealed MEMS device are those of the deposition chamber. Methods for sealing at higher pressures up to the order of atmospheric pressure and a few times that value, by the deposition of thin films, are however not widespread. Moreover, most of these atmospheric pressure techniques do not prevent material deposition inside the cavity. MEMS devices can be very fragile and deposition of material on the device is preferably avoided.
The openings which are typically produced in the sealing membrane using current approaches, have been etched, and are therefore sometimes quite large. Etching openings moreover requires patterning, masking steps etc which complicate the whole process of encapsulation. Such openings are provided mostly for allowing a sacrificial material etchant to reach out below the membrane or film, which had been supported temporarily by a sacrificial material, hereby dissolving or removing the sacrificial material and releasing the film, at least locally. The use of such a sacrificial material is state of the art when producing overhanging structures in packaging and MEMS processing technology.
There is a need for zero-level or wafer level packaging solutions which simplify the state of the art process sequences, and which moreover reduce the risk of having sealing material penetrating the area under the sealing membrane as for instance a cavity comprising for instance a fragile object as for instance a MEMS device.
It is known from US20040166606 to provide a structural material deposited on top of the sacrificial layer of a MEMS structure. It can be insulator or conductor and should have sufficient structural integrity so as to support the subsequent application of a liquid encapsulating material. It may be between 0.2-20 microns thick and has open regions to make a cage structure. These open regions are formed by removing material from the structural layer by sputtering, plasma etching, or wet etching. The size and spacing of the apertures should be large enough and/or spaced close enough such that the sacrificial layer can be later removed, but the apertures should be small enough as to not allow the protective material, such as Spin-On Glass (SOG), to encroach into the cavity and contact the movable structure. In addition, there should remain sufficient material to be structurally strong enough to not collapse upon application of the protective, encapsulating material. The sacrificial layer is then removed to create a microcavity in the space between the cage and the MEMS or micromachined device, using any of sublimation, sputter etching, ion beam milling, plasma etching or use of wet chemicals. Then the appropriate protective material is applied to encapsulate the MEMS device.
It is known from US2004/0224091 to provide sealing of a MEMS cavity by a nickel cap with pores which are sealed by reflow of Indium after removal of the sacrificial layer.
It is shown in US20040245586 to use various materials to encapsulate a MEMS structure, for example, silicon (polycrystalline, amorphous or porous, whether doped or undoped), silicon carbide, silicon-germanium, germanium, or gallium-arsenide.